An internal combustion engine has plugholes inserted with ignition coils. Each of the ignition coils includes a coil body constructed of a primary coil and a secondary coil. The coil body is provided with a plug cap formed of rubber. Each of the spark plugs includes an insulator fitted into a plug-fitting hole of the plug cap. The insulator has a tip end including a conductive terminal to be in contact with a secondary terminal (high voltage terminal), which is conductive with a high voltage winding end of the secondary coil, or a coil spring. The plug cap is provided for electrically insulating around the conductive terminal of the spark plug and protecting the conductive terminal from intrusion of water.
In this structure, the inner diameter of the plug-fitting hole of each plug cap is less than the outer diameter of the insulator of each spark plug for defining a predetermined fitting margin to produce the electrically insulation property and waterproof property. Accordingly, large force is applied for fitting the insulator into the plug-fitting hole by radially widening the plug cap.
According to JP-A-2005-190937, the spark plug has a structure facilitating the fitting of the sparkplug into the plug-fitting hole.
In JP-A-2005-190937, the sparkplug is partially surrounded with a joint of the plug cap. The joint has an inner periphery defining a corrugation including annular protrusions and recessions. The insulator of the sparkplug does not have a corrugation. In this structure, friction can be reduced in the fitting of the insulator into the joint.
However, the structure of the JP-A-2005-190937 is effective only when the insulator of the sparkplug does not have a corrugation.
Specifically, when the insulator, which has the corrugation, is fitted into the plug cap having the corrugation described in JP-A-2005-190937, waterproof property may be insufficient in a portion where the annular recession of the corrugation of the insulator is opposed to the annular recession of the corrugation of the joint of the plug cap. In addition, when, for example, the annular protrusion of the corrugation of the insulator is fitted to the annular recession of the corrugation of the plug cap, large force is needed for fitting of the insulator into the plug-fitting hole.
Accordingly, the fitting force needs to be further reduced at least when the sparkplug including the insulator having the corrugation is applied. Furthermore, the fitting force needs to be reduced regardless of providing of the corrugation to the insulator of the sparkplug.
For example, a cassette-type ignition coil device includes multiple ignition coils provided to a base bracket. The base bracket is mounted to the engine together with the ignition coils, so that the ignition coils can be simultaneously mounted to the engine, correspondingly to the number of the cylinders. In this structure, the fitting force becomes greater proportionally to the number of the cylinders. Accordingly, the fitting force needs to be further reduced in the cassette-type ignition coil device.